The rule book says "you MAY attempt to close a gate." During our game we had a card in play that said we could no longer seal gates. So I chose not to attempt to close the gate. What should happen at this point? Do I get sucked back through the gate or do I get a regular encounter? And yes, my character had already explored the Other World. Thank you in advance!

As long as you don't leave the location, you keep the Explored token. As long as you have the Explored token, you don't re-enter the gate. If you opt not to close the gate, your encounter is to do nothing at all. You can't get a random encounter or use a location special ability. Either close it and move on, or wait for the effect to go away and allow you to seal again.

Waiting at gates can be helpful sometimes, in case someone wants to off an Elder Sign or clue-generating item of some sort. Be careful how your phases play out though, so someone doesn't get drawn through a gate right before the other player seals it!